1. Field of the Invention
The present invention relates to a subscriber line terminal device connected to a subscriber line terminating device by an optical fiber, and in particular to a subscriber line terminal device, which selects a subscriber line terminal device monitoring signal or subscriber line terminating device alarm signal on the basis of order of priority, as well as to a subscriber line terminal device enabling a loopback test which includes a logical layer of the subscriber line terminal device.
Also, the present invention relates to a loopback test method for a subscriber line terminal device connected to a subscriber line terminating device by an optical fiber, and in particular to a loopback test method capable of a test including a logical layer of the subscriber line terminal device.
2. Description of the Related Art
Subscriber-based optical communication systems such as “Fiber to the Home” (FTTH), in which optical fiber is laid to a subscriber's home, are beginning to be adopted on a large scale.
FIG. 1A and FIG. 1B are block diagrams of the overall configurations of subscriber-based optical communication systems having subscriber line terminal devices (in-station media converter; hereafter called “in-station MC”) and subscriber line terminating devices (in-home media converter; hereafter called “in-home MC”). FIG. 1A shows an example in which one in-home MC 20 is connected to one in-station MC 1, and FIG. 1B shows an example in which, in order to increase user capacity, a plurality of in-home MCs 21 to 2n (where n is an integer greater than or equal to 2) are connected to one in-station MC 1.
The in-station MC 1 is installed in a central station, and the in-home MCs 20, 21 to 2n (hereafter, except when there is a special need to make distinctions, referred to generally as “in-home MCs 2”) are installed in user homes.
One of the ports of the in-station MC 1 and one of the ports of in-home MCs 2 are connected by means of single-mode optical fibers 60, 61 to 6n (hereafter, except when there is a special need to make distinctions, referred to generally as “optical fibers 6”). In the optical fibers 6, light of different wavelengths is allocated to downstream signals from the in-station MC 1 to in-home MCs 2, and to upstream signals from in-home MCs 2 to the in-station MC 1, to perform full-duplex bidirectional communication between MCs. For example, the wavelength 1.55 μm is allocated to downstream signals, and the wavelength 1.3 μm is allocated to upstream signals. By this means, point-to-point communication service is provided.
The other port of the in-station MC 1 is connected via a UTP (unshielded twisted pair) cable 5 to a WAN (wide-area network) 4. The other ports of the in-home MCs 2 are connected, via UTP cables 70, 71 to 7n (hereafter, except when there is a special need to make distinctions, referred to generally as “UTP cables 7”) to personal computers 30, 31 to 3n (hereafter, except when there is a special need to make distinctions, referred to generally as “PCs 3”) as examples of user terminals. By this means, data from the WAN 4 is transmitted to the PCs 3 via the in-station MC 1 and in-home MCs 2, and data from PCs 3 is transmitted to the WAN 4 via the in-home MCs 2 and in-station MC 1.
An optical ether access method is adopted between the in-station MC 1 and in-home MCs 2, and data signals (user data) is transmitted through media access control (MAC) frames conforming to the IEEE 802.3 standard.
On the other hand, the in-station MC 1 has an interface device which provides an interface with an operation system (OpS) terminal (not shown) for monitoring and control, and is connected to the OpS terminal via this interface device. The OpS terminal performs remote monitoring and remote control of in-home MCs 2 via the in-station MC 1. In order to perform the remote monitoring and remote control, control signals are transmitted from the in-station MC 1 to the in-home MCs 2, and monitoring signals are transmitted from the in-home MCs 2 to the in-station MC 1.
The control signals and monitoring signals are transmitted using specially defined frames (hereafter called “OAM (operation, administration and maintenance) frames”).
Control signals include “LOOP-MCQ” signals, as shown in FIGS. 3A and 3B, which request the initiation and cancellation of loopback tests (described below). Monitoring signals include “LOOP-MCS”, “RMT-POWER”, “RMT-FX-LINK”, “RMT-TX-LINK”, and “RMT-EQP” signals, as shown in FIGS. 3A and 3C.
The monitoring signal LOOP-MCS is a signal returned from an in-home MC 2 to the in-station MC 1 as the acknowledgement (initiation response or cancellation response) to the control signal LOOP-MCQ. The monitoring signal RMT-POWER is a signal indicating the state of power (normal/abnormal) of the in-home MC 2. The monitoring signal RMT-TX-LINK is a signal indicating the state of the UTP cable 7. The monitoring signal RMT-FX-LINK is a signal indicating the state of reception of optical signals of the optical fiber 6. The monitoring signal RMT-EQP is a signal indicating the state of the in-home MC 2. In the following, the monitoring signals that indicate the occurrences of abnormality may be called alarm signals.
These monitoring signals (alarm signals) are sent from in-home MCs 2 to the in-station MC 1, and from the in-station MC 1 to the OpS as the alarm sginals.
On the other hand, the in-station MC 1 monitors its own state, and when an abnormality is detected, sends an alarm signal to the OpS. As shown in FIG. 3D, there are signals “UPLINK”, indicating a link break in the UTP cable 5; “DOWNLINK”, indicating an optical link break in the in-station MC 1; and “EQP”, indicating equipment malfunction (memory error or similar) in the in-station MC 1.
As shown in FIG. 3E, alarms detected by the in-home MC 2 include “TXLINK”, indicating a link break in the UTP cable 7, and “FXLINK”, indicating an abnormality in reception of optical signals of the optical fiber 6.
Loopback tests are executed between the in-station MC 1 and in-home MCs 2, in order to confirm normality of the transmission line prior to the initiation of service when a new in-home MC 2 is to be installed in a user home, or in order to single out (narrow down) the fault point when a fault occurs. FIG. 13 is a sequence diagram of conventional loopback tests; FIG. 2 is a block diagram showing a logical hierarchy model of the in-station MC 1 and in-home MCs 2.
The in-station MC 1 has, as the logical hierarchy model, a physical layer (OSI layer 1) and logical layer (MAC layer (lower sublayer of OSI layer 2). An OAM sublayer for maintenance is provided in the physical layer. The logical layer is provided, in particular, in modes in which a plurality of in-home MCs 2 are connected to a single in-station MC 1.
A device to execute the processing of each of these layers is provided separately. For example, the PHY chip, which is an integrated circuit device, is a device which executes the processing of the physical layer excluding the OAM sublayer; the device which executes the processing of the sublayer comprises an ASIC (for example, FPGA), and the device which executes the processing of the logical layer comprises an MAC chip, which is an integrated circuit device.
Similarly, an in-home MC 2 has, for example, a PHY chip, which executes physical layer processing, an FPGA which executes OAM layer processing, and a MAC chip which executes logical layer processing. In some cases the in-home MC 2 does not have a logical layer; in such cases, the MAC chip may be omitted.
In the loopback test, first a loopback test initiation event is sent from the OpS terminal to the OAM sublayer via the interface device of the in-station MC 1. When the OAM sublayer of the in-station MC 1 receives the loopback test initiation event, the loopback test initiation request (LOOP-MCQ=1) shown in FIG. 3B is sent to the in-home MC 2 using an OAM frame. When the in-home MC 2 receives this request, it forms a loop path within its own data signal transmission pathway, and then sends the loopback test initiation response (LOOP-MCS=1) shown in FIG. 3C to the in-station MC 1 using an OAM frame. Then, a test frame in the format shown in FIG. 14 is transmitted and received continuously a plurality of times at a prescribed transmission period (interval) between the in-station MC 1 and in-home MC 2.
The test frame has a preamble, SFD (Start of Frame Delimiter), and test pattern. The test pattern is a PN pattern generated by a pseudo-noise (PN) generator (not shown) in the OAM sublayer (for example, PN15=X15+X2+1).
A test frame received by an in-home MC 2 is supplied to the OAM sublayer of the physical layer, and is then returned to the in-station MC 1.
The OAM sublayer of the in-station MC 1 checks the test pattern of the test frame returned from the in-home MC 2, and counts the number of error bits.
Transmission and reception of such test frames is repeated during a prescribed time period (time set by a timer), and when the OpS gives notification of the end of the test, the loopback test is ended. As a result of the test end, a loopback test cancellation request (LOOP-MCQ=0) shown in FIG. 3B is sent from the in-station MC 1 to the in-home MC 2 using an OAM frame, and a loopback test cancellation response (LOOP-MCS=0) shown in FIG. 3C is sent from the in-home MC 2 to the in-station MC 1 using an OAM frame.
As the test results, the in-station MC 1 notifies the OpS as to whether the PN synchronization was established and to the error count (total number of error bits). By this means, the connection of the physical layer including the optical fiber transmission path 6 is confirmed.
In the above-described remote monitoring of the prior art, all the monitoring signals RMT-EQP, RMT-POWER, and similar sent from an in-home MC 2 to the in-station MC 1 are supplied by the in-station MC 1 to the OpS, and in addition, the alarm signals UPLINK, DOWNLINK, and similar monitored by the in-station MC 1 are all supplied to the OpS.
Hence alarm signals giving notification of abnormalities occurring due to the same cause may be supplied to the OpS as a plurality of different alarm signals. For example, if a fault occurs in the optical segment between an in-home MC 1 and in-station MC 2, optical reception abnormality alarm signals RMT-FX-LINK and alarm signals DOWNLINK indicating a link break of the subscriber line port of the in-station MC 1 are collected by the in-station MC 1, and these are supplied to the OpS and displayed. And, when a power failure occurs in an in-home MC 2, the transmission optical power of the in-home MC 2 may fall, or the UTP-side link of the in-home MC 2 may be broken, so that the alarm signals DOWNLINK and RMT-TX-LINK are collected by the in-station MC 1, and the OpS is notified and results are displayed.
Thus in the prior art, alarm information collected by the in-station MC 1 is sent to and displayed at the OpS without modification, so that when a single alarm cause in an in-home MC 2 causes a plurality of alarm signals, it is not possible to identify the cause.
Further, in the conventional loopback tests, the test frame passes through the physical layer comprising the OAM sublayer of the in-station MC 1, but does not pass through the logical layer. Hence when an abnormality occurs in a logical layer device (for example, a MAC chip), this abnormality could not be detected by a loopback test. That is, when an abnormality occurs in a logical layer device, despite the fact that there is no abnormality in tests using test frames, in actual operation user frames cannot pass through the logical layer device, and so transmission of user frames is not possible.